David Maltby VC10 GPS question

Hi

In my day working on the VC10 we actually fitted the very first INS inertial nav systems on one for them for flight testing. The autopilot was very simple and and only a few buttons on the centre console!

I have been doing 'test flights' to see if my Brooklands runway was accurate enough to get a newly built '10' airborne with minimum fuel to enable a flight to Wisley.

I decided to do a few circuits and trim the aircraft - setting the GPS for the route direct to Wisley - EGTW. (The Wisley I have installed is a download from one of the sim websites not my own creation).

However activating the 'NAV' button on the AP it did not engage - so I checked the AP in HDG mode - perfect! According to Davids notes the 'NAV' button is set up purely as a 'GPS' on/off button for the VC10. The flight manual says that the Nav radio does not need to be switched on for this fucntion. So any ideas what I am doing wrong to not get 'on beam' using the FS GPS system?

Thank you.